Field of the Invention
This invention relates to manufacturing sodium hydroxide and ammonium sulfate or sulfuric acid by electrolysis using one or more semipermeable membranes. More particularly, the invention relates to a multicompartment electrolytic sodium sulfate splitter comprising an inert polymeric cation conductor and a polymeric anion conductor.
Sodium sulfate has been a major waste product of the pulp and paper industry for years. More recently, the destruction of certain chemical weapons has lead to a significant increase in dangerously contaminated Na.sub.2 SO.sub.4, which is a byproduct of the incineration of many of these weapons.
Sodium sulfate can be converted to sodium hydroxide and sulfuric acid by the reaction shown below: EQU Na.sub.2 SO.sub.4 (aq)+2H.sub.2 O(1).fwdarw.H.sub.2 SO.sub.4 (aq)+2NaOH(aq)
The electrolytic production of acids and alkalis from sodium sulfate solution dates back to 1923 when Harold Atwell investigated the practicality of recovering caustic soda and sulfuric acid of satisfactory concentration and purity by the electrolysis of sodium sulfate solutions. Most of the processes employed to date have been centered around the use of ion-permeable polymer membranes. Membrane electrolysis and bipolar membrane electrodialysis are possible routes for converting salts into their corresponding acids and bases.
The most direct process for producing sodium hydroxide from sodium sulfate is the electrolytic conversion of an aqueous solution of sodium sulfate into aqueous solutions of sulfuric acid and caustic soda. Numerous implementations of this process are known in the prior art. Most of them make use of electrolytic cells employing diaphragms or ion permeable membranes to separate the product solutions from the feed solutions, thus avoiding contamination of the products by the feedstock material.
Salt splitters of the prior art have utilized single compartments and multiple compartments. For example, U.S. Pat. No. 5,230,779 (Martin) teaches a two compartment salt splitter device which utilizes a selective ion exchange membrane.
In a two compartment salt splitter cell, an anion selective or cation selective membrane may be used to separate the anode from the cathode compartment. Where the sodium sulfate solution is fed depends on what membrane is used. When a cation selective membrane is used, the sodium sulfate solution will be fed into the anode compartment where it is converted to sulfuric acid and oxygen. The sodium atoms then migrate across the membrane to the cathode compartment where they combine with the hydroxyl ions to form sodium hydroxide. With an anion selective membrane, the sodium sulfate solution is fed into the anode compartment, and the sulfate ions and bisulfate ions migrate into the anode compartment where they combine with the hydroxide ions to form sodium hydroxide at the cathode compartment.
U.S. Pat. No. 2,829,095 discloses a process for the production of acidic and alkaline solutions by electrolysis of a salt solution in a multi-compartment electrolytic cell partitioned by a plurality of anion and cation exchange membranes. The patent also discloses the use of the process for direct production of sodium hydroxide and sulfuric acid from Glauber's salt (sodium sulfate decahydrate).
U.S. Pat. Nos. 3,135,573 and 3,222,267 claim a method and apparatus for converting aqueous electrolytic salt solutions to their corresponding acid and base solutions. A three or four compartment electrolytic cell separated by a cation exchange membrane and one or two porous, non-selective diaphragms is used for this purpose. When a solution of sodium sulfate is used as the salt solution, solutions of sodium hydroxide and sulfuric acid or sodium bisulfate are produced.
U.S. Pat. No. 3,398,069 claims a process for the electrolysis of an aqueous saline electrolyte in a multicellular device having cells separated by gas permeable electrodes and further partitioned by microporous fluid permeable diaphragms or ion-permselective membranes. When applied to a solution of sodium sulfate, the process produces solutions of sodium hydroxide and sulfuric acid.
U.S. Pat. No. 3,907,654 discloses an electrolytic cell particularly useful in electrolysis of sodium sulfate to form sulfuric acid and sodium hydroxide. The cell, which does not employ an ion permeable membranes, comprises a housing having a parent solution chamber and two electrode compartments located on the lower side of the housing and separated from each other but in communication with the parent solution chamber and two electrode compartments located on the lower side of the housing and separated from each other but in communication with the parent solution chamber and positioned vertically beneath or above. Mounted within the electrode compartments are an anode and a cathode, each of which is porous to permit passage of a product solution therethrough. The product solutions of sodium hydroxide and sulfuric acid separated by gravity forces are withdrawn through the porous electrodes.
U.S. Pat. No. 4,561,945 claims a process for producing sulfuric acid and caustic soda by electrolysis of an alkali metal sulfate in a three compartment membrane cell having a hydrogen depolarized anode. Hydrogen gas in the anode chamber is oxidized to produce hydrogen cations which migrate to the central (buffer) chamber through a membrane and combine with the sulfate anions from the alkali metal sulfate solution to produce sulfuric acid. Alkali metal ions are transported across another membrane to the cathode chamber to produce caustic and gaseous hydrogen. Both membranes used in the cell are cation selective membranes.
A similar process for increasing concentration of sulfuric acid in solutions containing an alkali metal sulfate, sulfuric acid and alkaline earth metal ions is disclosed in U.S. Pat. No. 4,613,416. Also in this case the anode compartment and the cathode compartment of a three compartment cell are each bounded by cation exchange membranes.
The development of ion selective membranes has also promoted the use of three compartment electrochemical cells partitioned by both cation and anion selective membranes. The use of such a cell for electrolysis of sodium sulfate has been disclosed, for example, by J. P. Millington ("An electrochemical unit for the recovery of sodium hydroxide and sulfuric acid from waste streams", in Ion-Exchange Membranes, D. S. Flett, Ed., Ellis Harwood Ltd. Publishers, Chichester, 1983, p.195).
In a similar manner, U.S. Pat. No. 5,098,532 (Thompson et al.) teaches a three chamber salt splitter device which utilizes ion selective polymers to separate the anode compartment and the cathode compartment from the sodium sulfate feed compartment. Thompson et al. teaches preferred cation exchange membranes are perfluorinated membranes such as Nafion and Flemion brand membranes, which are said to show good stability for sodium hydroxide concentration up to 50%.
Salt splitters of the prior art suffer from poor economics and less than optimum efficiency. In addition, corrosion during the electrolysis process has been a problem. Also, protons always manage to pass through the anion conducting polymer membrane material to some extent. It would therefore be desirable to have a salt splitter device which can solve these and other problems of the prior art devices.